Most parts of the country have been very dry for weeks so are you sure its rainwater? Might it be heater matrix failure - is your radiator level falling. If yes then you need a professional repair but get an independent garage to quote. If it is rainwater then first place I'd check are the membranes inside the doors -water is meant to run down inside the doors and membranes on the inner (cab) side of each door keep it from leaking in. These can be easily reattached. Or could be sunroof or windscreen seals allowing water in. Again professional repair probably needed. However its worth checking any drain channels in the engine bay near the foot of the windscreen for leaves or other stuff letting water pond up. Finally it could be the sealing on the bulkhead panel between the cab and the engine bay. My money would be on door membranes.

Over on the Corsa C forums they suggest first checking all drains at either end of scuttle panel where battery and washer bottle are fitted are clear and all leaves and other debris completely removed. There is some suggestion that the panel where the pedals are fixed to bulkhead can leak around its edges so worth investigating there as well.